The Faraday effect relates to rotation in the plane of polarization of linearly polarized light, that is light passed through matter in the direction of the lines of force of an applied magnetic field. The magnitude .alpha. of the rotation depends on the strength of the magnetic field H, the nature of the transmitting substance, the frequency v of the light, the temperature of the transmitting substance, and other parameters. In general,
.alpha.=VxH
where x is the length of the light path in the magnetized substance and V the so-called Verdet constant. The constant V is a property of the transmitting substance, its temperature, and the frequency of light.
Various optic fiber devices have been developed which exploit the Faraday effect for the measurement of electric current in a conductor. Such devices typically comprise one or more turns of optic fiber arranged in the form of a coil around the conductor. In accordance with the Faraday effect, the plane of polarization of linearly polarized light propogating in the optic fiber is rotated in proportion to the current passing through the conductor. Therefore, the electric current in the conductor can be calculated by measuring the degree of rotation of the linearly polarized light.
Both homodyne-based systems and heterodyne-based systems have been developed for measuring the degree of rotation of the linearly polarized light in a fiber optic sensor. Homodyne-based systems are disclosed in, for example, A. J. Rogers, IEEE J. Electron. Power Appl., 2, p. 20, 1977; A. M. Smith, Appl. Optic, 19, p. 3729, 1978. Heterodyne-based systems are disclosed in, for example, A. D. Kersey and D. A. Jackson, J. Lightwave Technology, Vol LT-4, No. 6, pp. 640-644 (June 1986).
Generally, fiber optic devices for measuring electric current employ specialty fibers, such as a helical core fiber (see M. P. Varnham et al., Digest ECOC/IOOC Conference Venice, p. 135, 1985), or a spun high-birefringence fiber (see L. Li et al., Electron. Lett., 22, p. 1142, 1986). Other special fibers include doped fibers with enhanced Verdet constants and annealed fibers which reduce both intrinsic birefringence and bend induced birefringence (see G. W. Day and S. M. Etzel, Digest ECOC/IOOC Conference Venice, p. 871, 1985).